1. Field of the Invention
The present invention relates to a semiconductor device temperature sensor and a semiconductor storage device and, more particularly, to a temperature sensor for a semiconductor storage device which needs a refresh action in order for a DRAM or the like to hold data.
2. Description of the Related Art
Conventionally, for example, a SRAM (Static Random Access Memory) has been used as a semiconductor storage device (memory) for portable systems including cellular phones. However, the required memory capacity tends to increase with e.g. improvement of the function of portable systems from year to year. Therefore, a DRAM (Dynamic Random Access Memory), which is a memory of large capacity, is going into use as a memory for portable systems. Then, the life time of a battery for portable systems becomes a subject.
The SRAM hardly consumes electric power for holding data, whereas the DRAM needs a refresh action on a regular basis to hold data and therefore it consumes a certain degree of electric power even when it is in a standby condition. In other words, with just holding data, a portable system with a DRAM adopted as a memory consumes electric power and decreases the capacity of a battery thereof even when it is not used actually.
To alleviate the decrease in battery capacity in such standby condition, an electric current consumption may be cut down by reducing the number of refresh actions of the DRAM in the standby condition. Concretely, for example, the DRAM has a characteristic such that the lower the temperature is, the longer the data-holding time is. Therefore, when the temperature of the DRAM is lower than a certain boundary temperature, the cycle of refresh actions (or time interval of refresh actions) may be set so as to be longer than that in a higher temperature condition thereby to reduce the number of refresh actions.
Incidentally, in the past there has been proposed a semiconductor storage device including: a refresh control circuit which switches the cycle of refresh actions of a memory cell; and a temperature-detecting unit which is biased with a bias voltage from a voltage-biasing unit including a reference unit and a regulator unit (refer, for example, to Patent Document 1). The semiconductor storage device is arranged so that it switches the control of the refresh control circuit in response to the detection of a predetermined temperature by the temperature-detecting part, and forces the control circuit to implement a refresh action of a short cycle in a high-temperature region and forces the control circuit to implement a refresh action of a long cycle in a low-temperature region, whereby the electric current consumption in the low-temperature region is reduced while the data-holding characteristic of the memory cell is maintained throughout a whole temperature range.
Further, in the past there has been proposed a programmable temperature sensor including: a programmable temperature-sensing unit which generates a forward sensing signal and a backward sensing signal, whose voltage levels depend on amounts of electric currents passing through a forward current source and a backward current source; and a comparator which compares the forward sensing signal and backward sensing signal in voltage level to generate a temperature information signal (refer, for example, to Patent Document 2). The programmable temperature sensor is arranged so that magnitudes of voltage levels of the forward sensing signal and backward sensing signal can be permutated at a predetermined critical temperature, which makes it possible to adjust the critical temperature from the outside.
Patent Document 1
Japanese Unexamined Patent Publication (Kokai) No. 2003-132678
Patent Document 2
Japanese Unexamined Patent Publication (Kokai) No. 2000-074748.
The prior art semiconductor device temperature sensor and semiconductor storage device and their problems will be described later in detail with reference to accompanying drawings.